One current trend in radio architecture for wireless transmitters in high data rate networks is using a dual-band power amplifier to simultaneously transmit data on two different frequency bands. It is well known that power amplifiers are non-linear devices. These non-linearities can manifest as intermodulation products that fall inside as well as outside the signal bandwidth. Depending on the severity of the non-linearity, the distortion can result in spectral components that violate spectral masks established by the Federal Communication Commission (FCC) and other regulatory bodies.
Generally, intermodulation products that fall on frequencies very far away from the signal's center frequency maybe removed by filtering. However, intermodulation products caused by third order non-linearities (IM3) of the power amplifier generally manifest as spectral components close to the center frequency of the signal. These spectral components cannot be easily removed by filtering. Therefore, alternate techniques are needed to remove or reduce these spectral components.
Digital predistortion is one technique used to compensate for the non-linearities of the power amplifier. The general principle behind digital predistortion is to distort the input signal in such a way that the overall system of the predistorter and the power amplifier is linear. In essence, the predistorter models the inverse non-linear characteristic of the power amplifier. As a general rule of thumb, the digital predistorter must support bandwidths that are three to five times the instantaneous signal bandwidth. The high sample rates required make digital predistortion impractical for dual-band signals. For instance, if the goal is to transmit a 20 MHz complex signal, the digital predistorter should run at a sampling rate greater than 100M samples/sec in order to effectively linearize the power amplifier. In the case of dual-band transmission where the signal bands are widely separated in frequency, very high sample rates are required because the total bandwidth of the combined signal is large.
As an example consider an LTE TDD implementation where one needs to transmit a 20 MHz signal (signal A) via band 35 (1850-1910 MHz) and simultaneously transmit another 20 MHz signal (signal B) via band 36 (1932-1990 MHz). Additionally, suppose signal A is centered on 1860 MHz and signal B is centered on 1980 MHz resulting in a total signal bandwidth of 140 MHz (=1990-1850 MHz). Using the rule of thumb, the predistorter will have to support a bandwidth in excess of 700 MHz (=5×140 MHz) for good linearization performance. Such a predistorter may be difficult to implement in current silicon technologies given the complexity of the design.
Another problem associated with multi-band transmission is that of insufficient linearization performance when the input signals are tuned to frequencies that are relatively nearby and when predistortion is being employed for linearization. In order to successfully linearize a power amplifier, the predistorter must be able to resolve and reduce or eliminate the effects of the non-linearity caused by the power amplifier. However, in the case of multi-band transmission, the signal energies may overlap after predistortion which would be impossible to resolve individually in the feedback path.